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Full text of "The Cambridge Natural History"

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vertebra there may be a ventral median process, arising of course
from the centrum, termed the hypapophysis.
From the sides of the neural arch, or from the centrum
itself, there is commonly a longer or shorter process on each
side, known as the transverse process. This is sometimes
formed of two distinct processes, one above the other; in
such cases the upper part is called a diapophysis, the lower
a parapophysis.
The neural arch may also bear other lateral processes, of
which one directed forwards is the nietapophysis, the other
directed backwards the anapophysis.
The series of bones which constitute the vertebral column
can be divided into regions. It is possible to recognise cervical,
dorsal, lumbar, sacral, and caudal vertebrae. In the case of
animals with only rudimentary hind-limbs, such as the Whales,
there is no recognisable sacral region. The neck or cervical
vertebrae are nearly always seven in number. The well-known
exceptions are the Manatee, where there are six, and certain
Sloths, where there are six, eight, or nine. These rare exceptions
only accentuate the very remarkable constancy in number, which
is very distinctive of the mammals as compared with lower
Vertebrata. There are of course abnormalities, the last cervical,
and sometimes the last two, assuming the characters of the
ensuing dorsals, by developing a more or less complete rib.
There are also recorded examples of Bradypus, in which the
number of cervicals is increased to ten. The characteristics,
then, of the cervical vertebrae are, in the first place, that they
do not normally bear free ribs, and that there is a break as a rule
between the last cervical and the first dorsal on this account. In
birds, for example, the cervicals, differing in number in different
families and genera, gradually approach the dorsals by the
gradually lengthening ribs. The transverse processes of the
vertebrae are commonly perforated by a canal for the vertebral
artery, and are bifid at their extremities. In some Ungulates
these vertebrae, moreover, approximate to the vertebrae of lower
"Vertebrata in the fact that there are ball and socket joints
between the centra, instead of only the fibrous discs of the
remaining vertebrae.
The first two vertebrae of the series are always very
different from those which follow. The first is termed the